1. Technical Field
The present invention relates to heating rollers and heating devices that are provided with the rollers, and to image forming apparatuses, and particularly relates to energy saving heating rollers, heating devices and imaging apparatuses that can warm up in a short time, and that can uniformly heat a material that is to be heated.
2. Description of the Related Art
Halogen lamp type fixing devices, which are one type of heating device, and which are constituted by arranging a halogen lamp within a fixing roller made of a hollow metal core of, for example, aluminum, wherein the fixing roller is set to a predetermined temperature by heating the halogen lamp, have conventionally been used as fixing devices. Since halogen lamps have been very easy and inexpensive to incorporate into fixing devices, they have conventionally been appropriate in many image forming apparatuses.
Furthermore, fixing devices have been widely used in recent color image forming apparatuses in recent years, and of these, oil-less color fixing systems in particular have been proposed with the object of, for example, simplifying the apparatuses and increasing the life of the rollers. These oil-less color fixing devices have generally been constituted by forming a thick rubber layer (2 to 3 mm) on the outside of the metal core of the upper and lower rollers, wherein the outermost layer is a releasing layer that has releasability.
However, in halogen lamp-type fixing devices, there has been a problem that the heating startup process is slow, and the warm-up time is long. Consideration has been given here to making the walls of the metal core of the fixing rollers thinner to achieve lower thermal capacity and to shorten the warm-up time. However with this, the deflection of the fixing rollers is too great, the pressure in the central part in the longitudinal direction is weak, and there is a limit to how far the thermal capacity can be reduced because of the risk of poor fixing defects. In particular, with roller-type fixing devices for color image forming apparatuses, there is a thick elastic layer of 1 to 3 mm on the outside portion of the metal core, and so even if the wall thickness of the metal core can be thinned and a reduction in the thermal capacity achieved, the time required for warming-up is still great since the thermal capacity of the elastic layer is large. Furthermore, poor fixing may occur if the thickness of the elastic layer is reduced, and thus there is a limit to how far the thickness of the elastic layer can be thinned down.
In recent years, heating devices in which the warm-up time is reduced in order to achieve energy savings, and energy saving thermal fixing devices, and the like, have been proposed (for example, see JP H8-129313A [referred to below as patent Reference 1] and JP H6-75493A [referred to below as patent Reference 2]).
The heating device disclosed in patent Reference 1 is provided with a heating roller has an internal elastic layer, on the outside of which a heat generating layer of a thickness of 10 to 150 μm is provided, wherein the heat generating layer is heated from the outside. The characteristics of this type are described below.
(1) The heating roller is formed from a thin, heat generating layer, so the thermal capacity can be made small. Thus, the warm-up time can be reduced. (2) The heat generating layer has an appropriate stiffness, and is fixed onto the elastic layer that is fixed on the metal core, so that it has superior durability. (3) There is an increased degree of freedom for selecting the width of the nip created due to the elasticity of the internal portion of the heating roller and the elasticity of the pressuring roller, so that the image forming apparatus is capable of high speed. (4) The pressuring deflection of the axles of the pressuring rollers is dampened by the elasticity of the inner portion of the heating roller, and a uniform nip width can be maintained in the longitudinal direction. Thus, the load on the heated material, such as transfer material is uniform, and problems such as waviness can be eliminated. (5) The appropriate stiffness of the heat generating layer acts to form a uniform nip width in the longitudinal direction. Thus, it is possible to prevent the heated material from curling due to the curvature of the heating roller at a nip portion that is flat by setting the surface hardness of the pressuring member to be similar to or greater than the surface hardness of the heating roller.
Furthermore, in patent Reference 2, there is disclosed a heating fixing device made of a heating roller containing an internal heater; and a flexible, deformable thin-walled tube-shaped body that is formed with thin walls and in a tube shape, and whose inner diameter is larger than the outer diameter of the heating roller, wherein the thin-walled tube-shaped body fits around the circumference of the heating roller in a free manner. With this type, since the apparent contact area (nip width) between the heating roller and the pressuring roller increases due to the presence of the thin-walled tube-shaped body, it is possible to form an apparently wide nip width even by the use of comparatively small diameter heating rollers and pressuring rollers by fitting movably the thin-walled tube-shaped body around the heating roller and creating contact with the pressuring roller. Thus it is possible to achieve a heating fixing device that is compact, and that consumes less electrical power.
However, in the heating device in the patent Document 1 described above, although it is possible to shorten the warm-up time to less than that when using a conventional halogen lamp, there is a problem in the loss of warm-up time since some of the heat that is generated in the heat generating layer escapes to the inside of the elastic layer.
Furthermore, in accordance with the above noted configuration, when paper containing an unfixed image passes between the rollers, the recording paper winds up onto the heating roller and it is exceedingly difficult to obtain releasability. In particular, when fixing unfixed color images onto thin paper, it is necessary to fix a maximum of four toner layers on top of each other, and the problem of releasability becomes more acute.
In the heating device according to the above-noted patent Reference 2, since the internal diameter of the thin-walled tube-shaped body is larger than the external diameter of the heating roller, there is an air layer between the thin-walled body and the heating roller. Thus there is a large thermal insulating effect and this is also advantageous to the warm-up time. However, since it is necessary to heat the roller from the inside, although originally it has been desirable to only heat the thin-walled tube-shaped body, there is a problem of very poor efficiency because the internal heating roller, which it is not necessary to heat, is heated, and the thin-walled tube-shaped body is indirectly heated using that conducted heat.